1. Field of Invention
The present invention relates to the field of electrically energized rail launchers, more specifically, an electromagnetic launcher/induction hybrid railgun with principle rail energization and fielding derived from a co-traveling energy pulse associated with a close-coupled parallel transmission line structure.
2. Description of Prior Art
There is an extensive history in regards these devices. At a very basic level, they are a linear electrical motor. Two conductors with an armature, free to move, providing the conduction connection between the rails. The approach is typically alternatively identified as a railgun or electromagnetic launcher. In order to drive an object of any reasonable mass to some useful velocity, typically very high currents in the multi mega-amp (MA) range are required in such a device. This requirement has specific practical problems that are inextricably associated with it and, to enumerate a few, consider the following.
Even though the rails may be of a very conductive material, short of superconducting, they will present with linearly increasing resistance as armature progresses along the rail. A net rail resistance of milli-Ohms (mΩ) would present a directly attributable voltage drop of approximately kilo-Volts for 1 MA and an associated, purely loss related power dissipation of ˜1 GW (Giga-Watt) or more.
If the rails were superconducting, the preceding would be avoidable, but there would be technical issues in regards preventing solid ice accumulation on rail surfaces. Aside from the running surface difficulty presented by on rail ice, moisture accretion within a high voltage environment is less than desirable. Short of effective sustained dry gas purging or in vacuum applications, this would be unavoidable.
Regardless of whether or not rails are superconducting or not, high current transfer from one rail, through armature to the other rail is invariably associated with some form of arcing behavior, either locally at the rail armature interface or across the entire rear surface of the armature (so called hybrid and plasma armatures). This results in rail erosion much as a welding arc would. In military applications, rail lifetime, as in the sense of conventional artillery barrel lifetimes, would not be a significant issue, as such systems are not subject to the sustained use expected of cost effective civilian systems.
Radiation from intense arcing can produce plasma ahead of the armature, which becomes a self limiting problem in that induced plasma serves to oppose armature acceleration as it represents a local, higher density gas component. Gas is of course associated with aerodynamic drag. This is sometimes called ablation resistance.
Radiation from intense arcing and typically associated with plasma formation can produce/induce secondary plasma emission at some other location on the rail; this can close as a switch (commutate) resulting in a secondary arc and power leakage.
An alternate type of armature is the liquid metal armature. This is an additional complication and has the potential to ‘freeze’ metal onto the rail with each event, thus affecting subsequent launch attempts.
In the simple, basic level, two rail configuration plus armature, given that the full drive current is channeled through the two parallel rails for the length of the barrel (acceleration region), it is then a basic fact that, as armature moves from the entrance to the end of rail, the circuital self-inductance increases commensurately, which complicates the issue of efficient power delivery to the armature. The simple railgun of this consideration is also, by convention, un-augmented. The entrance to the rail is equivalently the armature entry point to railgun acceleration region. The end of the rail is equivalently the armature exit point from railgun acceleration region. These armature entry and exit locations to a railgun acceleration region also correspond to rail element entry and end points. By common convention they may also be viewed as breech and muzzle locations.
U.S. Pat. No. 7,730,821 (Electromagnetic launcher with augmented breech) describes an electromagnetic launcher wherein a housing with breech and muzzle is slidably supported in a carriage supported on a trunnion. The first and second contacts are electrically connectable to receive electrical power from an electrical power supply. First and second augmentation conductors are disposed aft of the trunnion and are electrically connected to the first and second electrical contacts. First and second main conductors extend from the breech towards the muzzle. A current cross over connection is disposed towards the breech and electrically connects the first and second augmentation conductors with the first and second main conductors, respectively. The first and second electrical contacts and the first and second augmentation conductors may be engaged in slidable electrical contact over a portion of the first and second augmentation conductors, thereby accommodating recoil motion.
The primary deficiencies in regards conventional augmentation schemes are, correctly, in part, identified in the background of this patent, namely, significantly enhanced system inductance with associated motional back electromotive force (emf), which can be prohibitive for large aperture, hypervelocity systems. There are also the issues of energy storage in the augmenting system, losses and energy recovery at the very least.
The deficiencies of restricting oneself to the preceding general approach are well documented. As indicated, performance/efficiency and lifetime issues would be critical to any meaningful civilian application as distinct from military use.
An alternative with some merit, albeit less than ideal approach in some respects, is to be found in the so called STAR and UTSTAR railgun approach [The STAR Railgun Concept. I. R. McNab. IEEE Transactions on Magnetics. 35(1), 1999, 432-436]. A novel concept for a staged augmented railgun is evaluated in which an external field would be applied to the bore of the railgun in addition to the self induced magnetic field resulting from current flow in the main rails. The augmenting field would be provided by multiple saddle shaped coils, located on the exterior of the gun barrel. These electrical discharge energized magnetic coils, which would operate at high magnetic field strengths, would be transiently powered so that their magnetic field would only be present in the vicinity of the projectile. The magnetic field is alternatively defined to be the B field. The energy contained in the coils would be transferred forward to successive coils as the projectile travels down the barrel. In principle then energy losses would be minimized compared with other barrel concepts. However a more detailed evaluation of this concept shows that very high voltages could be needed to power the coils, so the concept may be unattractive for short barrels and high velocity applications.
There are a number of deficiencies apparent with this approach. Firstly, each coil has to be turned on rapidly, which has switching, timing and voltage implications for each coil assembly. This as alluded to in the statement that, high velocities may be unattractive with this concept. In addition, turning off the preceding coil precisely in sync with the coil turned on is just as significant an issue if one wishes to avoid any growth in system back electromotive force (by convention, emf) or odd induced transients courtesy of flux accretion in the volume defined by a common rail segment and armature. Indeed, since there is no explicit or even implicit reference to such a fundamental mitigating interaction, or its implications in the article, it is clear that the consequences of such behavior was not contemplated. The invention presented here naturally accomplishes this in a rail segment without any requirement for switching of an associated series array, in propagation direction, of individual coils.
The article, Launch to Space with an Electromagnetic Railgun [I. R. McNab. IEEE Transactions on Magnetics. 39(1), 2003, 295-304] describes many advances in electromagnetic (EM) railgun and power supply technology that had been made in recent years. Laboratory experiments with railguns have demonstrated muzzle velocities of 2-3 km/s and muzzle energies >8MJ. The extension of this technology to the muzzle velocities (>7500 m/s) and energies (>10GJ) needed for direct launch of payloads into orbit is very challenging, but may not be impossible. For launch to orbit, even long launchers (>1000 m) would need to operate at accelerations of >1000 gees to reach the required velocities, so that it would only be possible to launch rugged payloads, such as fuel, water and material. A railgun system concept is described here and technology development issues are identified. Estimated launch costs could be attractively low (<$600/kg) compared with the space shuttle (>$20000/kg), provided that acceptable launch rates could be achieved. Further evaluations are needed to establish the technical and economic feasibility with confidence.
This approach as articulated utilizes independent coil(s) transiently energized only when the projectile is nearby. Again, since there is no reference to induction behavior courtesy of synced coil switching on and off within a common rail element it is apparent that such a fundamental interaction/behavior was not contemplated here.
In addition and in support thereof, in terms of the description and graphics provided, it would appear to be the case that the UTSTAR and thus STAR modules are in fact indeed separate elements, each module with a separate current feed and an individual augmentation coil powered from power supplies positioned with each module as per the distributed energy store railgun with rail segments concept. This is probably only sensible if each module's rail elements are isolated from neighboring module rail elements, otherwise all neighboring rail elements would require rapid and significant opening switch capability on their own associated discharge power systems [illustrated in the previously cited I. R. McNab. IEEE Transactions on Magnetics. 35(1), 1999, page 434, FIG. 5]. Isolated module rail elements would naturally provide such. In other words, the system is in fact a series array of simple augmented railguns, in which case the local module is subject to the back emf problems associated with the local externally applied/augmented field armature interaction, as the turn off of the preceding module element's augmenting fields does not counterpoint the turn on of the augmenting fields in any other separate module. In effect, the flux linkage growth attributable to armature motion through impressed magnetic field is not counterpointed within any module.
As an indication that this interpretation is correct it should be noted that the author of the above two cited cases did not identify the fundamental fact that the appropriately synchronized and mutually matched turning on of an augmentation field, or flux linkage growth through armature motion in externally applied augmenting field, in conjunction with the turning off of the preceding augmentation field within a common rail, i.e. common circuit, element would in fact provide for matching forward emf with backward emf within this configuration and thus, essentially an induction drive/power transfer from the coils to the rail.
Further support for this understanding is to be found in the paper, Progress on Hypervelocity Railgun Research for Launch to Space by I. R. McNab, presented at the Electromagnetic Launch Technology, 2008 14th Symposium, in which research on critical issues for launch to space from a railgun carried on an airborne platform is considered; this includes techniques with the goal of achieving hypervelocity of ˜7 km/s: so far, 5.2 km/s has been achieved in a 7 m augmented railgun using a pre-injected plasma armature. Further study is being carried out on distributed power feed concepts that will improve the efficiency of launch for a long railgun: so far, 11 km/s has been achieved with a plasma arc in a five-stage system. Aerothermal behavior of a 10 kg projectile for flight from high-altitude launch into orbit is being investigated: so far, the results show that an acceptable amount (˜15 mm) of nose tip ablation will occur.
The configuration presented is possessed of a unitary augmenting field structure for the given rail element. It is further apparent that within this technical approach, the externally applied magnetic field will in general not be optimal within the duration of the armature acceleration event. This is deficient relative to the invention presented here whereby, when appropriately configured, the co-traveling magnetic field pulse experiences only limited diminution during armature acceleration per rail segment. It is similarly deficient in that the back emf in this case would be approximately 300 to 400 and more percent greater than presented by this invention whereby only the rail discharge self-back emf applies, free of augmenting field contribution. The latter may not be significant for small bore, moderate velocity systems, but will be significant to prohibitive at larger bore and higher velocities and thus presents a scalability issue.
The author also did not provide any explanation of how to transfer the coil energy to successive coils along the barrel in a manner matching projectile motion.
Finally, if the objective was to accelerate a projectile to +20 km/s the switching and timing requirements and the associated voltages and currents of a sequenced individual coil series array would become severe. This is no different to the problem presented by the coil switching issue in pure induction accelerators such as coil guns, where the voltage required increases with velocity and thus high voltage commutation of multiple coils in axial series represents the technological limit to achievable launch velocity.